Blue eyes are typically associated with beauty, or perhaps Frank Sinatra. But to University of Wisconsin anthropologist John Hawks, they represent an evolutionary mystery.
For nearly all of human history, everyone in the world had brown eyes. Then, between 6,000 and 10,000 years ago, the first blue-eyed baby was born somewhere near the Black Sea.
For some reason, that baby's descendants gained a 5 percent evolutionary advantage over their brown-eyed competitors, and today the number of people with blue eyes tops half a billion.
"What does it mean?" said Hawks, who studies the forces that have shaped the human species for the last 6 million years.
Nobody knows. It is one of the unanswered questions about evolution that persist 200 years after the birth of Charles Darwin, whose birthday will be celebrated worldwide Thursday.
Darwin amassed a lifetime of observations on plants and animals to famously conclude that all life on Earth evolved from simple organisms through a painstakingly slow process of tiny, random changes and a continuous contest for survival of the fittest.
Though Darwin published his masterwork, On the Origin of Species, 150 years ago and died in 1882, studies on evolution continue apace. Much of that effort focuses on the species Darwin considered the pinnacle of the evolutionary process: Homo sapiens.
Until recently, conventional wisdom held that human beings had mastered their environment so thoroughly that the imperative to evolve in many ways diminished roughly 10,000 years ago, when agriculture gave rise to more-stable societies.
"People thought that with technology and culture, there'd be no reason for physical things to make any difference," said Milford Wolpoff, a paleoanthropologist at the University of Michigan. "If you can ride a horse, it doesn't matter if you can run fast." That turned out to be wrong. As it happens, the pace of evolution has been speeding up - not slowing down - in the 40,000 years since our ancestors are thought to have fanned out from what is now Ethiopia to populate the globe.
And in the 5,000 to 10,000 years since agriculture fostered the growth of large societies, the pace has accelerated to 100 times historical levels.
"When there's more people, there are more mutations," Wolpoff said. "And when there are more mutations, there's more selection."
Hawks and other scientists quantified this in late 2007 by comparing more than 3 million genetic variants in the DNA of 269 people of African, European, Asian and Native American descent. They created sophisticated computer models to scour the As, Cs, Gs and Ts of the genome in search of telltale patterns that signal recent adoption of favorable genes.
Their methods rely on the observation that new mutations are not inherited alone, but are passed along in large DNA chunks. Over time, random changes make those chunks smaller. By comparing the length of those chunks in different people, scientists can estimate how long the beneficial mutation has been spreading through the gene pool.
The analysis turned up about 1,800 genes - 7 percent of the genome - that have been widely adopted in the past 40,000 years. Researchers using more conservative methods estimate the number of such genes at 300 to 500.
The function of most genes is unknown. The scientists identified 17 genes for hair cells that are also involved in hearing, and which may have been under selective pressure because they help people identify voices or speak tonal languages such as Mandarin.
And they found a number of genes involved in brain development, including a version of a dopamine receptor gene called DRD4 that is sweeping through the European population. Some think it is a novelty-seeking variant, others that it affects libido. What they do know is that having two copies increases the odds of attention deficit hyperactivity disorder.
Among the genes whose purpose is understood, the biggest category is devoted to fighting infectious diseases. For instance, the researchers found more than a dozen new genetic variants involved in fighting malaria spreading rapidly among Africans.
Scientists had previously identified several mutations that offered protection against the disease. Most were shared by people of African descent, since the scourge is most widespread on that continent. But malaria afflicts people throughout the tropics and subtropics, and additional mutations to combat the disease arose in Thailand and New Guinea, Hawks said.
One of the newly discovered mutations helps defend against a form of the disease in which malaria parasites congregate in blood cells in the placenta, causing a high rate of miscarriage.
Diet is another big force behind recent human evolution. As humans made the transition from being hunter-gatherers to farmers, their bodies had to adapt to new kinds of foods.
The best-known example involves the gene that regulates a person's ability to make an enzyme required to digest lactose, the sugar in milk. Historically, the LCT gene shut down in early childhood as babies were weaned off breast milk. But after cows, sheep and goats were domesticated, people with a mutation that allowed them to drink milk as adults had a nutritional advantage that made it easier for them to propagate their genes.
DNA analyses have shown that the mutation cropped up in Europe about 8,000 years ago, and quickly spread all the way to India. Today, it is carried by more than 95 percent of people of northern European descent.
A 2007 study bolsters the theory that the rise of pastoralism prompted the gene's spread. Using new techniques to analyze ancient DNA, German and British researchers checked the genes of eight European farmers who lived 7,000 to 7,800 years ago, before the widespread adoption of a herding lifestyle. None of those early farmers had the mutation for lactose tolerance.
The adaptation was so important, it happened at least five times. Hawks and colleagues have recently discovered LCT variants that arose independently over the past 5,000 years among herders living in the Arabian Peninsula and sub-Saharan Africa.